Description of the German bunker constructions

Fa – Feldmaßige Ausbau (Field-type constructions)
Mainly constructed from locally available materials; lumber, sand, gravel, turf and rocks. Concrete was also used if available. A wall and ceiling thickness of 40 to 60 cm was thought and a compound construction where e.g. a double timber wall lined with rocks was often used. The floor would be approx. 20 cm thick. The construction was intended to be able to stop small-arms fire, shrapnel and light debris. 
Vf – Verstärkt Feldmaßig (Reinforced field-type constructions)
These were bunkers, carried out as concrete or concrete reinforced with steel bars, with wall and ceiling up to 100 cm thickness and a floor up to 40 cm. The construction was designed to protect its inhabitants from the effects of a 50 kg aerial bomb dropped on the roof or a direct wall hit from a 105 mm artillery shell. Some of these designs became designated types in the Bauform catalogue.
St – Ständig (Permanent constructions)
The Regelbau concept embraced only permanent constructions which came in multiple “Baustärke” (building-strengths). These types were all made of reinforced concrete, or reinforced concrete with an outer casing of Formsteine (concrete pre-fab blocks), or combinations of both. Most common is Baustärke B (building-strength) featuring ceiling and wall dimensions of min. 200 cm and a floor of 80 cm. This construction was sufficient to protect from the effects of a half-ton aerial bomb on the roof or a hit from a 220 mm artillery shell. If local conditions required an even sturdier construction, you could go to Baustärke A, comprising walls and ceiling of 350 cm and capable of withstanding the heaviest artillery of the day and aerial bombs up to 1000 kg.A St-bunker was, if used for habitation or as a fighting post, secured against gas warfare, had its own air-filtering installation (manual and/or electrical powered), telephone, radio, heating, lighting and other facilities. Some had their own well for water supply. The bunkers have compartments sealed with gas proof steel (or sometimes concrete) doors. Every St bunker had at least two ways of exiting, either by two entrances or by one entrance and an emergency exit. The St bunkers also have pressure valves in every compartment.
Sk – Sonderkonstruktion (Special constructions)
For special applications for which no standard type of bunker was available, special bunkers were designed. These bunkers were designed for the special purpose and/or location and were often based on St-bunker conditions (but sometimes also built in Vf strength) and built mostly once and therefore unique.
Regelbau prefix: 
Prefix German name Englisch name
R Heer Army
M Kriegsmarine mittlere Navy medium
S Kriegsmarine schwere Navy heavy
FL Kriegsmarine Flak Navy Anti Aircraft
L Luftwaffe Airforce
V Versorgung Support
Ausbaustärken – Strengths expansion in the Atlantikwall:
Type Name Cover Wall Hight
Fa Feldmassige Ausbau <0.60 <0.60 1.90 mtr
Vf Verstarkt Feldmassig 0.30 - 1.00 0.30 - 1.00 2.10 mtr
St Standig (B-standaard) 2.00 2.00 2.20 mtr
St Standig (A-strengthened) 3.50 3.50 2.20 mtr
Sk Sonderkonstruktion > 1.50 >1.50 >1.50 mtr
Bauprogramm – Building program „Neue Westwall/Atlantikwall“:
Period Called
04.1940 - 11.1941 From surveillance to stationary defense
11.1941 - 09.1942 Anfang bau „Neue Westwall“ from Norway - France
09.1942 - 04.1943 Winterausbauprogramm. Start of the name „der Atlantikwall“
05.1943 - 09.1943 2. Bauprogramm
10.1943 - 04.1944 2. Winterausbauprogramm
05.1944 - 09.1944 Sommerausbauprogramm
10.1944 - 05.1945 Weitere Ausbau